Duret's right to a degree. Best bet for older conversion stuff is to search for Specialized and Trek frames when they were still cranking out steel. A lot of those earlier models had horizontal dropouts. You'll probably have to deal with a 1" threaded headset, but for your purposes it probably doesn't matter. I have a late 80s lower end Specialized (read: heavy) setup as my townie this way.

I've also got a dirt commuter I built recently. Fixed gear 29er built around a Qball frame. Moderately priced, hand built, 4130 steel.